1. Field of Invention
The field of the currently claimed embodiments of this invention relates to methods and systems for evaluating epileptic zones of a subject's brain.
2. Discussion of Related Art
Epilepsy is one of the most common brain disorders, characterized by chronically recurrent seizures resulting from excessive electrical discharges from groups of neurons [1]. Epilepsy affects about 50 million people worldwide and approximately 40% of all individuals with epilepsy have intractable seizures, which cannot completely be controlled by medical therapy [2-4]. That is, seizures continue to occur despite treatment with a maximally tolerated dose of at least two anti-epilepsy drugs (AEDs). The direct cost of assessing and treating patients with medically refractory epilepsy (MRE) ranges from $3-4 billion annually ($16 billion in direct and indirect costs) in the US (based on a 1996 publication) [5]. 80% of these costs are accounted by patients whose seizures are not adequately controlled by AEDs [6]. The burden of MRE, however, is much greater than heavy financial costs. MRE is a debilitating illness where individuals lose their independence, causing profound behavioral, psychological, social, financial and legal issues [7-11]. Recurrent seizures impair socialization and psychological development during formative years and may lead to an inability to obtain an education, gainful employment, or driving privileges. The development of a learned helplessness and low self-esteem can worsen as long as epilepsy is intractable. Cognitive performance may be impaired by MRE as well as by side effects of AED therapy [7-11].
Surgical Treatment of MRE
Despite the heavy sequelae from MRE, there is a potentially curative procedure—surgical resection of the epileptogenic zone (EZ), which is the minimal area of brain tissue responsible for generating the recurrent seizure activity [12]. However, to be effective, this procedure depends on correct identification of the EZ, which is often unclear. A comprehensive pre-surgical evaluation is necessary to pinpoint the EZ as well as to identify the risk of neurologic morbidity such as visual impairment. Various non-invasive and invasive methods are used. Non-invasive techniques include scalp EEG, video-EEG, neuropsychological tests, speech-language studies, and brain imaging (MRI, PET, Ictal SPECT). Of these methods, the highest predictor of surgical success is identification of a single visible MRI lesion [13], yet despite the advances in imaging technologies, a significant number of surgical patients with focal epilepsy (˜25%) continue to have normal MRIs [14-17].
When the less invasive methods fail to identify the EZ, the method of last resort is an invasive evaluation, comprising placement of subdural grid electrodes (SDE) or stereotactically placed depth electrodes (stereoelectroencephalography, a.k.a. SEEG) and subsequent prolonged extra-operative monitoring in a dedicated Epilepsy Monitoring Unit (EMU) [41]. Subdural grids and strips are the most common invasive method used in the United States [18, 41-43, 46]. Despite the high spatial resolution provided by the subdural methodology, which allows for accurate mapping of superficial cortical areas, relatively deep epileptic foci cannot be sampled with adequate spatial and temporal resolution. In addition, subdural grids require relative large craniotomies and are, in general, limited to exploration of one hemisphere. There thus remains a need for improved systems and methods for evaluating epileptic zones of a subject's brain.